Count the Number of Special Characters II solution and explanation

def count_special_characters(word):
    seen = {}
    special_set = set()
    for i, c in enumerate(word):
        if c.islower():
            if c not in seen:
                seen[c] = i
        elif c.isupper():
            lower = c.lower()
            if lower in seen and lower not in special_set and seen[lower] < i:
                special_set.add(lower)
    return len(special_set)

import java.util.*;

public class SpecialCharacters {
    public static int countSpecialCharacters(String word) {
        Map<Character, Integer> firstLowerIndex = new HashMap<>();
        Set<Character> specialSet = new HashSet<>();

        for (int i = 0; i < word.length(); i++) {
            char c = word.charAt(i);
            if (Character.isLowerCase(c)) {
                firstLowerIndex.putIfAbsent(c, i);
            } else if (Character.isUpperCase(c)) {
                char lower = Character.toLowerCase(c);
                if (firstLowerIndex.containsKey(lower) && firstLowerIndex.get(lower) < i && !specialSet.contains(lower)) {
                    specialSet.add(lower);
                }
            }
        }
        return specialSet.size();
    }
}

function countSpecialCharacters(word) {
    let firstLowerIndex = {};
    let specialSet = new Set();

    for (let i = 0; i < word.length; i++) {
        let c = word[i];
        if (c === c.toLowerCase()) {
            if (!(c in firstLowerIndex)) {
                firstLowerIndex[c] = i;
            }
        } else {
            let lower = c.toLowerCase();
            if (lower in firstLowerIndex && firstLowerIndex[lower] < i && !specialSet.has(lower)) {
                specialSet.add(lower);
            }
        }
    }
    return specialSet.size;
}

#include <iostream>
#include <vector>
#include <unordered_set>

int countSpecialCharacters(std::string word) {
    std::vector<int> lastLower(26, -1);
    std::vector<int> lastUpper(26, -1);
    std::unordered_set<char> specialSet;

    for (int i = 0; i < word.length(); i++) {
        char c = word[i];
        if (islower(c))
            lastLower[c - 'a'] = i;
        else
            lastUpper[c - 'A'] = i;
    }

    for (int i = 0; i < 26; i++) {
        if (lastLower[i] != -1 && lastUpper[i] != -1 && lastLower[i] < lastUpper[i]) {
            specialSet.insert('a' + i);
        }
    }
    return specialSet.size();
}

#include <stdio.h>
#include <stdbool.h>

int countSpecialCharacters(const char* word) {
    int last_index[52];
    for (int i = 0; i < 52; i++) last_index[i] = -1;
    int count = 0;
    bool found_special[26] = { false };

    for (int i = 0; word[i] != '\0'; i++) {
        char c = word[i];
        int idx = (c >= 'a') ? c - 'a' : c - 'A' + 26;
        last_index[idx] = i;
    }

    for (int i = 0; i < 26; i++) {
        int lower_idx = last_index[i];
        int upper_idx = last_index[i + 26];
        if (lower_idx != -1 && upper_idx != -1 && lower_idx < upper_idx) {
            found_special[i] = true;
            count++;
        }
    }
    return count;
}

using System;
using System.Collections.Generic;

public class SpecialCharacterCounter {
    public static int CountSpecialCharacters(string word) {
        int[] lastLower = new int[26];
        int[] lastUpper = new int[26];
        Array.Fill(lastLower, -1);
        Array.Fill(lastUpper, -1);
        HashSet<char> specialSet = new HashSet<char>();

        for (int i = 0; i < word.Length; i++) {
            char c = word[i];
            if (char.IsLower(c)) {
                lastLower[c - 'a'] = i;
            } else {
                lastUpper[c - 'A'] = i;
            }
        }

        for (int i = 0; i < 26; i++) {
            if (lastLower[i] != -1 && lastUpper[i] != -1 && lastLower[i] < lastUpper[i]) {
                specialSet.Add((char)('a' + i));
            }
        }
        return specialSet.Count;
    }
}

function numberOfSpecialChars(word: string): number {
    const first: number[] = Array.from({ length: 'z'.charCodeAt(0) + 1 }, () => 0);
    const last: number[] = Array.from({ length: 'z'.charCodeAt(0) + 1 }, () => 0);
    for (let i = 0; i < word.length; ++i) {
        const j = word.charCodeAt(i);
        if (first[j] === 0) {
            first[j] = i + 1;
        }
        last[j] = i + 1;
    }
    let ans: number = 0;
    for (let i = 0; i < 26; ++i) {
        if (
            last['a'.charCodeAt(0) + i] &&
            first['A'.charCodeAt(0) + i] &&
            last['a'.charCodeAt(0) + i] < first['A'.charCodeAt(0) + i]
        ) {
            ++ans;
        }
    }
    return ans;
}

func numberOfSpecialChars(word string) (ans int) {
	first := make([]int, 'z'+1)
	last := make([]int, 'z'+1)
	for i, c := range word {
		if first[c] == 0 {
			first[c] = i + 1
		}
		last[c] = i + 1
	}
	for i := 0; i < 26; i++ {
		if last['a'+i] > 0 && first['A'+i] > 0 && last['a'+i] < first['A'+i] {
			ans++
		}
	}
	return
}